Systems and methods for high-contrast, near-real-time acquisition of terahertz images

ABSTRACT

A cw terahertz image beam is upconverted by a nonlinear optical process (e.g., sum- or difference-frequency generation with a near IR cw upconverting beam). The upconverted image is acquired by a near IR image detector. The bandwidths and center wavelengths of the terahertz image beam and the upconverting beam are such that wavelength filtering can be employed to permit an upconverted image beam to reach the detector while blocking or substantially attenuating the upconverting beam.

This application is a continuation-in-part of U.S. non-provisionalapplication Ser. No. 14/561,141 filed Dec. 4, 2014 in the names ofVladimir G. Kozlov and Patrick F. Tekavec (now U.S. Pat. No. 9,377,362),which in turn claims benefit of (i) U.S. provisional App. No. 61/912,004filed Dec. 4, 2013 in the names of Vladimir G. Kozlov and Patrick F.Tekavec, and (ii) U.S. provisional App. No. 62/007,904 filed Jun. 4,2014 in the names of Vladimir G. Kozlov and Patrick F. Tekavec. Each oneof said applications is hereby incorporated by reference as if fully setforth herein.

FIELD OF THE INVENTION

The field of the present invention relates to imaging usingterahertz-frequency radiation. In particular, systems and methods aredisclosed for high-contrast, near-real-time acquisition of terahertzimages.

BACKGROUND

A number of systems and methods for generation, detection, or imagingwith terahertz-frequency radiation have been disclosed previously. Someof those are disclosed in:

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No. 2012/0008140 entitled “Terahertz sensing system        and method” published Jan. 12, 2012 in the names of Khan et al        (Massachusetts Institute of Technology; now U.S. Pat. No.        8,514,393 issued Aug. 20, 2013);    -   U.S. Pat. No. 8,599,474 entitled “Alignment and optimization of        a synchronously pumped optical parametric oscillator for        nonlinear optical generation” issued Dec. 3, 2013 to Kozlov et        al (Microtech Instruments);    -   U.S. Pat. No. 8,599,475 entitled “Alignment and optimization of        a synchronously pumped optical parametric oscillator for        nonlinear optical generation” issued Dec. 3, 2013 to Kozlov et        al (Microtech Instruments);    -   U.S. Pat. No. 8,599,476 entitled “Alignment and optimization of        a synchronously pumped optical parametric oscillator for        nonlinear optical generation” issued Dec. 3, 2013 to Kozlov et        al (Microtech Instruments);    -   Wu et al; “Two-dimensional electro-optic imaging of THz beams”;        Applied Physics Letters Vol. 69 No. 8 p. 1026 (1996);    -   Jiang et al; “Terahertz imaging via electrooptic effect”; IEEE        Transactions on Microwave Theory and Techniques Vol. 47 No.        12 p. 2644 (1999);    -   Jiang et al; “Improvement of terahertz imaging with a dynamic        subtraction technique”; Applied Optics Vol. 39 No. 17 p. 2982        (2000);    -   Nahata et al; “Two-dimensional imaging of continuous-wave        terahertz radiation using electro-optic detection”; Applied        Physics Letters Vol. 81 No. 6 p. 963 (2002);    -   Sutherland et al; Handbook of Nonlinear Optics 2ed (2003); New        York: Marcel Dekker;    -   Yonera et al; “Millisecond THz imaging based on two-dimensional        EO sampling using a high speed CMOS camera”; Conference on        Lasers and Electro-Optics, Paper No. CMB3 (2004);    -   Ding et al; “Phase-Matched THz Frequency Upconversion in a GaP        Crystal”; Conference on Lasers and Electro-Optics, Paper No.        CTuL3 (2006);    -   Ding et al; “Observation of THz to near-Infrared parametric        conversion in ZnGeP2 crystal”; Optics Express Vol. 14 No. 18 p.        8311 (2006);    -   Hurlbut et al; “Quasi-Phasematched THz Generation in GaAs”;        Conference on Lasers and Electro-Optics, Paper No. CTuGG (2006);    -   Cao et al; “Coherent detection of pulsed narrowband terahertz        radiation”; Applied Physics Letters Vol. 88 p. 011101 (2006);    -   Vodopyanov; “Optical generation of narrow-band terahertz packets        in periodically inverted electro-optic crystals: conversion        efficiency and optimal laser pulse format”; Optics Express Vol.        14 No. 6 p. 2263 (2006);    -   Lee et al; “Generation of multicycle terahertz pulses via        optical rectification in periodically inverted GaAs structures”;        Applied Physics Letters Vol. 89 p. 181104 (2006);    -   Khan et al; “Optical detection of terahertz radiation by using        nonlinear parametric upconversion”; Optics Letters Vol. 32 No.        22 p. 3248 (2007);    -   Schaar et al; “Intracavity terahertz-wave generation in a        synchronously pumped optical parametric oscillator using        quasi-phase-matched GaAs”; Optics Letters Vol. 32 No. 10 p. 1284        (2007);    -   Khan et al; “Optical detection of terahertz using nonlinear        parametric upconversion”; Optics Letters Vol. 33 No. 23 p. 2725        (2008);    -   Vodopyanov et al; “Resonantly-enhanced THz-wave generation via        multispectral mixing inside a ring-cavity optical parametric        oscillator”; Conference on Lasers and Electro-Optics, Paper No.        CTuG1 (2009);    -   Pedersen et al; “Enhanced 2D-image upconversion using        solid-state lasers”; Optics Express Vol. 17 No. 23 p. 20885        (2009).    -   Hurlbut et al; “THz-wave generation inside a high-finesse        ring-cavity OPO pumped by a fiber laser”; Conference on Lasers        and Electro-Optics, Paper No. CWF3 (2010);    -   Tekavec et al; “Efficient high-power tunable terahertz sources        based on intracavity difference frequency generation”; Paper No.        IRMMW-THz in 36th International Conference on Infrared,        Millimeter and Terahertz Waves (2011); and    -   Tekavec et al; “Terahertz generation from quasi-phase matched        gallium arsenide using a type II ring cavity optical parametric        oscillator”; Proc. SPIE 8261, Terahertz Technology and        Applications V, 82610V;    -   doi:10.1117/12.909529 (2012);    -   Clerici et al; “CCD-based imaging and 3D space-time mapping of        terahertz fields via Kerr frequency conversion”; Optics Letters        Vol. 38 No. 11 p. 1899 (Jun. 1, 2013);    -   Fan et al; “Room temperature terahertz wave imaging at 60 fps by        frequency up-conversion in DAST crystal”; Proc. 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SUMMARY

A terahertz image beam is upconverted by a nonlinear optical process(e.g., sum- or difference-frequency generation with a visible or near IRupconverting beam). The upconverted image is acquired by a visible ornear IR image detector. The terahertz image beam and upconverting beamcomprise continuous-wave (cw) beams. The bandwidths and centerwavelengths of the terahertz image beam and the upconverting beam aresuch that wavelength filtering can be employed to permit an upconvertedimage beam to reach the detector while blocking or substantiallyattenuating the upconverting beam.

Objects and advantages pertaining to upconversion of terahertz imagesand detection of the upconverted images may become apparent uponreferring to the exemplary embodiments illustrated in the drawings anddisclosed in the following written description.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the disclosed or claimed subject matter, nor is itintended to be used as an aid in determining the scope of the claimedsubject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a first example of an apparatus foracquiring an upconverted terahertz image.

FIG. 2 illustrates schematically a second example of an apparatus foracquiring an upconverted terahertz image.

FIGS. 3A-3D and 3F are examples of spectra of an upconverting opticalbeam and upconverted terahertz image beams, wherein (i) both beamscomprise trains of femtosecond pulses (FIG. 3A), (ii) the terahertzimaging beam is a cw beam and the upconverting beam is a train ofnanosecond pulses (FIG. 3B), (iii) both beams comprise continuous-wave(cw) beams (FIGS. 3B and 3F), or (iv) both beams comprise trains ofpicosecond pulses (FIGS. 3C and 3D), before maximal polarization- orwavelength-based filtering; FIG. 3E is an example of polarization- andwavelength-filtered spectra of an upconverted terahertz image beam and aresidual upconverting optical beam using picosecond pulses.

FIG. 4 is a table comparing estimated signal strength of severaltechniques for acquiring a terahertz image using trains of nanosecond,picosecond, or femtosecond pulses, or continuous-wave (cw) beams.

FIG. 5 illustrates schematically an example arrangement of a cwterahertz imaging beam reflected from a sample and upconverted by a cwupconverting beam.

FIGS. 6A-6C are visible images of three test objects; FIGS. 7A-7C areraw upconverted terahertz images of those objects in transmission; FIGS.8A-8C are normalized, upconverted terahertz images of those objects intransmission.

FIGS. 9A-9C are visible images of three other test objects; FIGS.10A-10C are raw upconverted terahertz images of those objects intransmission; FIGS. 11A-11C are normalized, upconverted terahertz imagesof those objects in transmission.

FIG. 12 illustrates schematically another example of an apparatus foracquiring an upconverted terahertz image using cw terahertz imaging beamand a cw upconverting beam.

The embodiments depicted are shown only schematically: all features maynot be shown in full detail or in proper proportion, certain features orstructures may be exaggerated relative to others for clarity, and thedrawings should not be regarded as being to scale. The embodiments shownare only examples: they should not be construed as limiting the scope ofthe present disclosure or appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of the present disclosure and appended claims, andregardless of use in any cited or incorporated references listed herein,the terms “continuous-wave” and “cw,” when NOT followed by the term“modelocked,” shall denote an optical or terahertz beam wherein theaverage power and the peak power are substantially equal to each other.In other words, a continuous-wave or cw beam does not comprise a trainof pulses having peak power higher (often orders of magnitude higher)than the average power. Conversely, the terms “continuous-wavemodelocked” and “cw modelocked” shall denote an optical or terahertzbeam that comprises a train of pulses having peak power higher thanaverage power and typically having a pulse repetition rate thatcorresponds to a round trip time of an optical resonator cavity.

The terahertz (THz) wave region of the electromagnetic spectrum (i.e.,about 0.05 THz to about 10 THz), a relatively under-developed spectral“gap” between the microwave and long-wave infrared spectral regions, isinteresting for several reasons. Many biological and chemical compoundshave unique absorption features in this spectral region, makingterahertz radiation attractive for imaging in defense, security,biomedical, and industrial settings. Terahertz radiation can pass withlittle or no attenuation through many substances that are opaque tooptical, ultraviolet, or infrared radiation (e.g., ceramics, fabrics,dry organic materials, plastics, paper, or various packaging materials)with little attenuation. Imaging with terahertz radiation enablessub-millimeter spatial resolution, potentially providing higher qualityimages compared to images obtained at longer wavelengths (e.g., usingmillimeter waves).

Direct acquisition or detection of images at terahertz frequencies ishampered by the typically low sensitivity or low spatial resolution ofsuitable detectors (e.g., bolometer, Golay cell, or microbolometerarray), by the need for raster scanning to obtain a two-dimensionalimage if a single-channel detector is used, or by the need for cryogeniccooling of a bolometric detector or array. Sensitive two-dimensionaldetector arrays with high spatial resolution operable at roomtemperature (e.g., CCD arrays, CMOS arrays, or InGaAs arrays) arereadily available for detecting images in the visible and near infrared(near IR) portions of the electromagnetic spectrum (i.e., wavelengthsfrom about 400 nm to about 3000 nm); the problems noted above for directdetection of terahertz-frequency images could be avoided by using suchdetectors, however, those detectors are not sensitive to terahertzradiation. Various nonlinear optical effects can be exploited to enableuse of visible or near IR detectors or arrays for acquisition ofterahertz images.

So-called coherent detection can be employed for acquiring terahertzimages using a visible or near IR detector; examples are disclosed inthe references of Wu et al, Yonera et al, Jiang et al, and Zhang et al(cited above). The coherent detection method typically employs a shortoptical pump pulse (e.g., <100 femtoseconds (fs) at a visible or near IRwavelength) to create a broadband THz pulse. Coherent detection of theTHz pulse can be achieved by mixing it with a short optical probe pulse(e.g., <100 fs at a visible or near IR wavelength; typically anamplitude-scaled replica of the pump pulse) in an electro-optic crystal.The polarization of the optical probe pulse is rotated by the THz pulseelectric field due to the Pockels effect; the amount of rotation isproportional to the THz field amplitude and can be measured by detectionthrough an analyzer polarizer. Coherent detection can be implementedusing a single detector element combined with raster scanning of theobject or the THz image, or a visible or near IR detector array can beemployed (e.g., a CCD camera or CMOS array), thereby eliminating theneed for raster scanning. However, image contrast of the acquired imagestypically is limited by a low signal to noise ratio. In addition, thebroad optical frequency bandwidth (typically about 2 to 3 THz) of theTHz radiation generated by the short optical pulses often results insignificant atmospheric absorption of certain frequencies within thatbandwidth, resulting in loss of THz power and distortion of the THzfrequency spectrum.

Disclosed herein is an alternative to coherent detection, in which avisible or near IR detector or array can be employed for acquiring THzimages by (i) nonlinear optical upconversion of those images to opticalor near infrared wavelengths (i.e., about 400 nm to about 3000 nm) and(ii) detection of the upconverted images using the detector or array.Examples are illustrated schematically in FIGS. 1 and 2 of systems forgenerating and acquiring upconverted terahertz images. In each example,an object 10 is illuminated by a beam of terahertz radiation (i.e.,terahertz imaging beam 21) at a wavelength of λ_(THz)=c/ν_(THz) (c isthe speed of light). A terahertz image can be generated by reflection orscattering from the object 10 or by transmission through or around theobject 10. The reflected or transmitted terahertz image beam 20 iscollected by a first focusing element 31 (shown as a single lens inFIGS. 1 and 2; an off-axis parabolic reflector or other one or moretransmissive or reflective focusing elements suitable for terahertzradiation can be employed) and relayed to an upconverting nonlinearoptical medium 36. An upconverting beam 22 at a visible or near IRwavelength λ_(UC) is combined (usually substantially collinearly) withthe terahertz image beam 20 by a beam combiner 34; the beam combiner canbe of any suitable type or construction (e.g., a pellicle), and caneither reflect the upconverting beam 22 while transmitting the terahertzimage beam 20 (as shown in FIGS. 1 and 2), or can transmit theupconverting beam 22 while reflecting the terahertz image beam 20 (notshown).

The terahertz image beam 20 and upconverting beam 22 co-propagatethrough the upconverting nonlinear optical medium 36, in which one ormore upconverted image beams 24 are produced by nonlinear opticalinteractions (sum- or difference-frequency generation; SFG or DFG,respectively) between the terahertz image beam 20 and the upconvertingbeam 22. Residual radiation from the upconverting beam 22 is attenuatedor blocked by one or more wavelength dependent filters 38 or one or morepolarizers 39 (which collectively constitute an image filteringelement). The one or more upconverted image beams 24 (at1/λ_(DFG)=1/λ_(UC)−1/λ_(THz) or 1/λ_(SFG)=1/λ_(UC)+1/λ_(THz) or both)are collected by a second focusing element 32 (shown as a single lens inFIGS. 1 and 2; any one or more transmissive or reflective focusingelements suitable for the wavelength(s) of the upconverted image beam(s)can be employed) and relayed to a visible or near IR detector array 40for detection of the upconverted image. Whether any residual radiationfrom the terahertz image beam 20 reaches the detector array 40 islargely irrelevant, because the terahertz radiation typically would haveno discernible effect on the visible or near IR detector array 40.However, the detector array 40 is sensitive to residual radiation fromthe upconverting beam 22; any such residual upconverting radiationreaching the detector array 40 represents an undesirable backgroundsignal for detection of the one or more upconverted image beams 24(further discussed below).

A detector array 40 is shown and described in the examples, enablingacquisition of entire images by receiving simultaneously differentspatial portions of the upconverted image beam on multiple correspondingdetector elements of the detector array. However, the present disclosureor appended claims also can encompass use of a single detector elementscanned across the upconverted image beam so as to receive sequentiallydifferent spatial portions of the upconverted image beam on the singledetector element.

The effective focal length (e.g., the focal length of a single lens orsingle curved mirror, or the effective focal length of a multicomponentfocusing element) of the first focusing element 31 is f₁; the effectivefocal length of the second focusing element 32 is f₂. In theconfiguration of FIG. 1, the distance between the object 10 and thefirst focusing element 31 is d_(o1), the distance between the firstfocusing element 31 and the nonlinear optical medium 36 is d_(i1), andthe object 10, the first focusing element 31, and the nonlinear opticalmedium 36 are positioned so that 1/d_(o1)+1/d_(i1)=1/f₁, i.e., theobject 10 and the nonlinear optical medium 36 are positioned atconjugate planes defined by the focusing element 31 so that a terahertzimage of the object 10 is formed at the nonlinear optical medium 36 witha magnification of −d_(i1)/d_(o1). That terahertz image is upconvertedby SFG or DFG with the upconverting beam 22 in the nonlinear opticalmedium 36. The distance between the nonlinear optical medium 36 and thesecond focusing element 32 is d_(o2), the distance between the secondfocusing element 32 and the detector array 40 is d_(i2), and thenonlinear optical medium 36, the second focusing element 32, and thedetector array 40 are positioned so that 1/d_(o2)+1/d_(i2)=1/f₂, i.e.,the nonlinear optical medium 36 and the detector array 40 are positionedat conjugate planes defined by the focusing element 32 so that theupconverted image generated in the nonlinear optical medium 36 isreimaged at the detector array 40 with a magnification of−d_(i2)/d_(o2). The overall magnification of the image formed on thedetector array 40 relative to the object 10 is(d_(i1)·d_(i2))/(d_(o1)·d_(o2)).

In the configuration of FIG. 2, the distance between the object 10 andthe first focusing element 31 is f₁, and the distance between the firstfocusing element 31 and the nonlinear optical medium 36 is also f₁. As aresult, a spatial Fourier transform of the terahertz image is formed atthe nonlinear optical medium 36; it is that spatial Fourier transformthat is upconverted by SFG or DFG with the upconverting beam 22 in thenonlinear optical medium 36 to generate upconverted spatial Fouriertransform(s) of the terahertz image. The distance between the nonlinearoptical medium 36 and the second focusing element 32 is f₂, and thedistance between the second focusing element 32 and the detector array40 is also f₂. As a result, an upconverted image is formed at thedetector array 40 from the upconverted spatial Fourier transformgenerated in the nonlinear optical medium 36. The overall magnificationof the image formed on the detector array 40 relative to the object 10is −(λ_(UC)·f₂)/(λ_(THz)·f₁). The configuration of FIG. 2 can in someinstances lead to a more compact arrangement of the image upconversionsystem, because often d_(i1)+d_(i2)+d_(o1)+d_(o2) is larger than2·(f₂+f₁).

In any real system the locations of the object 10, the focusing elements31 and 32, the nonlinear optical medium 36, or the detector array 40might deviate from the exact positions given for the two configurationsdescribed above. For the purposes of the present disclosure or appendedclaims, a given imaging arrangement shall be considered to conform toone of those configurations if an upconverted image is formed at thedetector array 40 of sufficiently good quality for a given application.

In the two configurations described above, an upconverting focusingelement 33 conveys the upconverting beam 22 into the nonlinear opticalmedium 36 to interact with the terahertz image beam 20. The upconvertingbeam 22 preferably is made as small as practicable at the nonlinearoptical medium 36 (for increased intensity of the upconverting beamresulting in increased upconversion efficiency) while stillsubstantially overlapping spatially the entire terahertz image beam 20and exhibiting a substantially flat wavefront and sufficiently smallspatial intensity variation across the spatial extent of the terahertzimage or Fourier transform. To those ends, typically the focusingelement 33 (e.g., a single lens, a single curved mirror, a telescope, ora suitable combination of one or more transmissive or reflectivefocusing components) is arranged to form a relatively gently focusedbeam waist of the upconverting beam 22 at the nonlinear optical medium36. For example, a focusing element 33 can be arranged to produce a beamwaist about 7 mm wide (full width at half maximum, i.e., FWHM) at thenonlinear optical medium 36; other suitable widths can be employed. Theeffect of an upconverting beam size that is too small depends on theconfiguration of the imaging system. In the configuration of FIG. 1, asmall upconverting beam 22 can result in loss of peripheral portions ofthe upconverted image if peripheral portions of the terahertz image arenot upconverted. In the configuration of FIG. 2, a small upconvertingbeam 22 can result in loss of sharpness of the upconverted image iflarger wavevector components (i.e., peripheral portions of the spatialFourier transform) of the terahertz image are not upconverted. In eitherconfiguration, deviations from a flat wavefront or uniform intensity ofthe upconverting beam 22 typically can be tolerated; the magnitude ofsuch deviations that can be tolerated can vary and typically isdependent on the image quality needed or desired for the upconvertedimage.

Examples are shown in FIGS. 3A-3F of wavelength spectra of theupconverting beam 22 and one or two upconverted image beams 24. In eachexample one or both upconverted image beams 24 are formed in thenonlinear optical medium 36 by one or both of sum- anddifference-frequency generation (SFG and DFG, respectively) between theTHz image beam 20 (centered at ν_(THz)≈1.55 THz) and the upconvertingbeam 22 (centered at λ_(UC)≈800 nm in FIG. 3A; centered at λ_(UC)≈1064nm in FIGS. 3B-3E; centered at λ_(UC)≈1550 nm in FIG. 3F). Depending onthe nature of the SFG and DFG nonlinear optical processes, in someinstances only one of those processes will produce a correspondingupconverted image beam 24.

In previous examples exhibiting spectra similar to those of FIG. 3A(e.g., as disclosed in the above-cited references of Wu et al, Yonera etal, Jiang et al, and Zhang et al), the upconverting beam 22 comprises atrain of pulses about 100 fs in duration with a corresponding spectralbandwidth of about 15 nm centered at λ_(UC)≈800 nm. The upconvertedimage beams 24 have corresponding center wavelengths of λ_(SFG)≈796 nmand λ_(DFG)≈804 nm with similar spectral bandwidths. In this example theupconverted image beams 24 are polarized orthogonally relative to theupconverting beam 22 due to the nature of the nonlinear optical process(e.g., Type I or Type II nonlinear optical processes) employed for SFGand DFG in the nonlinear optical medium 36. In addition to the spatialoverlap of the terahertz image beam 20 and the upconverting beam 22discussed above, substantial temporal overlap of the respective pulsetrains of those beams is also necessary for achieving a desiredefficiency of upconversion of the terahertz image. A suitable delay lineis inserted into the beam paths of one or both of the terahertz imagebeam 20 or the upconverting beam 22; the delay line can be adjustable toenable optimization of the upconversion efficiency. Shorter pulseduration (ca. 100 fs) enhances the efficiency of the SFG and DFGprocesses, but the concomitant larger bandwidth (ca. 15 nm) causessubstantial spectral overlap of the upconverting beam 22 and theupconverted image beams 24. Because of that overlap, a wavelengthdependent filter 38 typically cannot be employed as part of the imagefiltering element for attenuating the residual upconverting beam 22.Orthogonal polarization of the upconverting beam 22 and the upconvertedimage beams 24 enables use of a polarizer 39 as the image filteringelement for attenuating the residual upconverting beam 22. However, apolarizer will at best exhibit attenuation of about 10⁻⁶ for the blockedpolarization state (10⁻⁴ to 10⁻⁵ is more realistic) and the residualupconverting beam 22 typically is not in a pure linear polarizationstate due to passage through the nonlinear optical medium 36 and variousother optical components. The portion of the residual upconverting beam22 that leaks through the polarizer 39 often can be substantially moreintense than the upconverted image beams 24. In addition, the broadoptical frequency bandwidth of the THz image beam 20 suffers fromsignificant atmospheric absorption of certain frequency components, asnoted above. For all these reasons, pulses of such short duration (a fewhundred femtoseconds or shorter with correspondingly large spectralbandwidth) are not particularly well suited for upconversion ofterahertz images.

In various other previous examples (e.g., those disclosed in thereferences of Khan et al, Nahata et al, Cao et al, and Ding et al, citedabove; representative spectra shown in FIG. 3B with λ_(UC)≈1064 nm,λ_(SFG)≈1058 nm, and λ_(DFG)≈1070 nm), a continuous-wave terahertz beam,and an upconverting beam that comprises pulses that are severalnanoseconds (ns) in duration with correspondingly narrow spectralbandwidth (e.g., <0.1 nm), are employed, thereby enabling a wavelengthdependent filter to be employed in the image filtering element forattenuating residual upconverting radiation before detection of theupconverted signal. However, the longer pulses require pulse energy ofthe upconverting beam to be close to the damage threshold of thenonlinear optical medium 36 to achieve detectable upconversion of theterahertz image. Such pulse energies typically are only available in lowrepetition rate pulsed lasers (e.g., pulse repetition rates on the orderof 10 Hz), however, pulse-to-pulse fluctuations tend to obscuredetection of the small signal level of the upconverted image. Mostdetector arrays are sensitive to average power, which is quite low atsuch low repetition rates. The repetition rate is also comparable todesired frame rates for near-real-time video imaging and is thereforenot well-suited to that application; video-rate imaging would require asingle shot per frame. In addition, the upconverting beam typically mustinclude radiation at the desired DFG wavelength to enable detection ofthe upconverted image, making that detection an inherentlynonzero-background process (e.g., as in the reference of Cao et al). Forall of these reasons, pulses of such long duration (several nanosecondsor longer with correspondingly narrow spectral bandwidth) and such largepulse energy are not particularly well suited for upconversion ofterahertz images.

In an example disclosed by Kozlov et al (US 2015/0153234; examplespectrum shown in FIG. 3C), the terahertz image beam 20 and theupconverting beam 22 comprise trains of pulses about 6-10 picoseconds(ps; FWHM) in duration, the upconverting beam is about 0.3 nm inbandwidth (FWHM), and the terahertz image beam is similarly narrow inits frequency spectrum (e.g., less than 100 GHz (FWHM) centered at about1.55 THz, thus enabling substantial avoidance of atmospheric absorptionbands). With the upconverting beam 22 centered at λ_(UC)≈1064 nm, theupconverted image beams 24 have corresponding center wavelengths ofλ_(SFG)≈1058 nm and λ_(DFG)≈1070 nm and similarly narrow spectralbandwidths. As in the previous example, the upconverted image beams 24are polarized orthogonally relative to the upconverting beam 22 due tothe nature of the nonlinear optical process (e.g., Type I or Type IInonlinear optical processes) employed for SFG and DFG in the nonlinearoptical medium 36. Orthogonal polarization of the upconverting beam 22and the upconverted image beams 24 enables use of a polarizer 39 in theimage filtering element for attenuating the residual upconverting beam22. Longer pulses relative to the example of FIG. 3A results in reducedpeak intensities and reduced efficiency of the SFG and DFG processes,although those processes are still more efficient than in the example ofFIG. 3B. However, the correspondingly smaller spectral bandwidthsubstantially eliminates spectral overlap of the upconverting beam 22and the upconverted image beams 24, enabling use of one or morewavelength dependent filters 38 in the image filtering element, insteadof or in addition to polarizer 39, for attenuating the residualupconverting beam 22. A combination of one or more wavelength dependentfilters 38 and the polarizer 39 can conservatively yield attenuation ofthe residual upconverting beam 22 on the order of 10⁻⁸ and perhaps asmuch as 10⁻¹⁰ or 10⁻¹². Alternatively, the lack of spectral overlap ofthe upconverting beam 22 with the upconverted image beams 24 can enableelimination of the polarizer 39 from the image filtering element and useof alternative, potentially more efficient, nonlinear optical processesin the nonlinear optical medium 36, e.g., a Type 0 nonlinear opticalprocess wherein all polarizations are parallel to one another. The pulseduration also enables terahertz image acquisition to be combined withterahertz tomographic techniques to acquire images originating fromdiffering depths within a sample with spatial resolution on the order ofseveral millimeters.

Another example disclosed by Kozlov et al (FIG. 3D) is similar to thatof FIG. 3C, except that the pulses employed are about 1-2 ps in duration(FWHM) with a bandwidth of about 1 nm (FWHM). Those parameters canincrease the efficiency of the terahertz image upconversion (higherintensity due to shorter pulse duration) while still enabling effectivewavelength-based filtering of the residual upconverting beam. Theshorter pulse duration also enables improved spatial resolution (e.g.,on the order of a millimeter) when terahertz image acquisition iscombined with terahertz tomographic techniques to acquire imagesoriginating from differing depths within a sample.

In another example disclosed by Kozlov et al, the terahertz image beamcan be centered at about 0.85 THz with spectral width similar to one ofthe preceding examples (enabling substantial avoidance of atmosphericabsorption bands). If the upconverting beam is centered at aboutλ_(UC)≈1064 nm with similar spectral width, the upconverted image beamswill have corresponding center wavelengths of λ_(SFG)≈1061 nm andλ_(DFG)≈1067 nm and similar spectral widths. The smaller spectralseparation between the upconverting beam and the upconverted image beamsmay require a polarizer or enhanced spectral filtering for adequateattenuation of the upconverting beam.

In an inventive example according to the present disclosure, both theterahertz imaging beam 21 (and therefore also the terahertz image beam20) and the upconverting beam 22 are continuous-wave (cw) beams (FIGS.1, 2, and 5). Use of cw beams would at first appear to be undesirable;pulsed beams are typically employed (as in the previous examplesdisclosed above) to exploit the concomitant higher peak powers to drivethe nonlinear optical upconversion process. However, higher power cwterahertz sources (e.g., producing average power on the order of 0.1 Wto 1 W over frequency ranges from about 0.05 THz to about 0.4 THz, up toabout 1.5 THz, or up to about 3 THz) can be employed that deliver powerin the cw terahertz image beam 20 comparable to peak powers in theprevious examples. However, the much higher duty cycle of the cwterahertz image beam 20 and the upconverting beam 22 (i.e., unity for cwbeams versus on the order of 10⁻⁴ for picosecond pulses at typicalmodelocked source repetition rates) results in comparable or largeraverage upconverted average power in the upconverted image beam 24.Imaging detectors 40 typically employed are sensitive to average powerin the upconverted image beam 24. An example apparatus is schematicallyillustrated in FIG. 5, wherein a cw terahertz source 100 produces theterahertz imaging beam 21 and a cw visible or near-IR source 200produced the upconverting beam 22. In the examples of FIGS. 1 and 2, theterahertz image is formed by transmission through or around the object10; in the example of FIG. 5, the terahertz image is formed byreflection or scattering from the object 10. Typical spectra wouldresemble the examples of FIG. 3B (ν_(THz)≈1.55 THz, λ_(UC)≈1064 nm,λ_(SFG)≈1058 nm, and λ_(DFG)≈1070 nm) or FIG. 3F (ν_(THz)≈0.3 THz,λ_(UC)≈1550 nm, λ_(SFG)≈1547.6 nm, and λ_(DFG)≈1552.5 nm). The averageterahertz power and upconverting beam powers of the continuous-wavebeams 20 and 22 result in sufficient power in the upconverted image beam24 (see the table of FIG. 4) to enable near-real-time terahertz imaging,e.g., video-rate terahertz imaging at frame rates of about 5-30 FPS ormore. It has been observed that the upconverted image signal variessubstantially linearly with terahertz imaging beam power and withupconverting beam power, without evidence of saturation. This suggeststhat further increases in upconverted image signal can be achieved byfurther increasing terahertz and upconverting beam powers.

Any suitable source 100 of the continuous-wave (cw) terahertz imagingbeam 21 can be employed that produces sufficient power over the desiredterahertz frequency range. Some examples exploit the so-calledbackward-wave oscillator (BWO) effect, and include Terasource tubesproduced by Terasense® Group, Inc. Such sources can provide terahertzaverage power from about 0.1 W up to about 1.0 W over terahertzfrequencies from about 0.08 THz up to about 0.36 THz; other suitablesources can be employed. A BWO-type or other terahertz source can becombined with a terahertz amplifier of any suitable type (extant orfuture-developed; e.g., an 0.85 THz amplifier developed by NorthropGrumman) to provide higher upconverting average power. One or moreterahertz sources can be used in combination with one or more frequencydoublers or triplers (or both) to extend the accessible terahertzfrequency range, e.g., as disclosed in U.S. Pat. No. 8,035,083 issued toKozlov et al, which is incorporated by reference. Those examples or anyother suitable continuous-wave terahertz sources, now extant orfuture-developed, can be employed as the cw terahertz source 100 withinthe scope of the present disclosure or appended claims.

Any suitable source 200 of the visible or near infrared upconvertingbeam 22 can be employed that produced sufficient power at the desiredupconverting wavelength and with sufficiently small upconvertingbandwidth. Typical sources 200 include solid state, semiconductor, orfiber lasers operating in the visible or near infrared. Examples includeKoheras® BOOSTIK single-frequency fiber lasers available from NKTPhotonics A/S, which can produce up to about 15 W of upconvertingaverage power at upconverting wavelengths between about 1030 nm andabout 1090 nm, or up to about 10 W of upconverting average power atupconverting wavelengths between about 1530 nm and about 1575 nm. Thoseexamples or any other suitable continuous-wave visible or near infraredsources, now extant or future-developed, can be employed as the cwupconverting source 100 within the scope of the present disclosure orappended claims.

Corresponding upconverting bandwidths can be as small as a few tens ofkHz in the example upconverting sources 200 described above. Typicallythe upconverting bandwidth is less than about 0.1 nm, which correspondsto about 30 GHz for λ_(UC)≈1000 nm or about 20 GHz for λ_(UC)≈1500 nm.The upconverting bandwidth therefore can impose a lower limit on theterahertz frequency employed while keeping the upconverting beam 22 andthe upconverted image beams 24 spectrally separated. However, even ifthe upconverting bandwidth is exceedingly narrow (as with the examplesources described above) so that the upconverting spectrum and theupconverted image spectra do not overlap and are completely separated,their close spacing resulting from use of relatively low terahertzfrequencies (e.g., separation of only about 2 nanometers or less forterahertz frequencies less than about 0.3 THz) can nevertheless resultin insufficient (or at least problematic) rejection of the upconvertingbeam 22 while transmitting one or both upconverted image beam(s) 24, orinsufficient transmission of the upconverted image beams 24 whilesubstantially rejecting the upconverting beam 22 (discussed furtherbelow).

Any suitable nonlinear optical medium 36 can be employed for generatingthe upconverted image beam(s) 24. One suitable medium comprises a stackof two or more optically contacted gallium arsenide (GaAs) or galliumphosphide (GaP) plates. The thickness of the plates is selected toresult in quasi-phase-matched upconversion by the upconverting beam 22of the terahertz image beam 20 to the one or more upconverted imagebeams 24. In one example, a stack of 6 to 12 GaAs plates, each about 300μm thick, can be employed to produce the upconverted image beams 24 at1058 nm and 1070 nm from the terahertz image beam 20 at about 1.55 THzand the upconverting beam 22 at about 1064 nm, with the polarization ofupconverting beam 22 substantially orthogonal to that of upconvertedbeam(s) 24. More plates can result in higher upconversion efficiency,but the difficulty of maintaining sufficiently high optical qualityincreases with increasing numbers of plates. In another example, asingle GaAs plate up to about 8 to 9 mm thick (i.e., less than or aboutequal to the coherence length for the upconversion process) can beemployed to produce the upconverted image beams 24 at about 1547.6 andabout 1552.4 from the terahertz image beam 20 at about 0.3 THz and theupconverting beam 22 at about 1550 nm and polarized orthogonally withrespect to the upconverted image beams 24. Other plate numbers orthicknesses can be employed for other combinations of terahertzfrequency and upconverting wavelength. Any other suitable nonlinearoptical material(s) can be employed, any other suitable phase-matchingor quasi-phase-matching schemes can be employed, and any suitablenonlinear optical process, e.g., Type 0, I, II, and so forth, can beemployed.

If the nonlinear optical process in the medium 36 produces only oneupconverted image beam 24, or if only one of multiple upconverted imagebeams 24 is desired to be detected at the detector array 40, then ashort-pass or long-pass cutoff filter 38 can be employed that attenuatesor blocks the upconverting beam 22 while enabling at least a portion(spectrally) of one upconverted image beam to reach the detector 40. Forexample, a long-pass filter 38 with a cutoff wavelength between theupconverting wavelength and the difference-frequency wavelength can beemployed that would attenuate or block the residual upconverting beam 22at the upconverting wavelength and the upconverted image beam 24 at thesum-frequency wavelength (if present), but would transmit to thedetector array 40 at least a portion of the upconverted image beam 24 atthe difference-frequency wavelength; an example of a spectrumtransmitted by such an arrangement is shown in FIG. 3E for one of theexamples disclosed by Kozlov et al. Similarly, a short-pass filter 38with a cutoff wavelength between the upconverting wavelength and thesum-frequency wavelength could be used to enable the upconverted imagebeam at the sum-frequency wavelength to reach the detector array 40while attenuating or blocking the residual upconverting beam 22 at theupconverting wavelength and the upconverted image beam 24 at thedifference-frequency wavelength (if present).

In another example, a so-called notch filter 38 (e.g., a Bragg filter ora multilayer thin film interference-type filter) nominally centered atthe upconverting wavelength could be employed to attenuate or block theresidual upconverting beam 22 while enabling at least portions(spectrally) of both upconverted beams 24 to reach the detector 40. Inpractice, a thin film notch filter suitable for the particularcombination of wavelengths shown in FIG. 3B, 3C, 3D, or 3F may notprovide sufficient discrimination between the upconverting beam 22 andupconverted image beam 24, i.e., currently it is difficult to design andmanufacture such a thin film notch filter with a sufficiently narrowrejection bandwidth that exhibits both sufficient attenuation of theresidual upconverting beam 22 and sufficient transmission of theupconverted image beams 24 at those wavelengths. Also, depending on thenature of the source of the upconverting beam 22, its spectrum can insome instances exhibit excess bandwidth or unwanted sidebands; thatissue can in some instances be mitigated by use of a bandpass filter(used in transmission) or a notch filter (used in reflection) centeredat λ_(UC) to “clean up” the spectrum of the upconverting beam 22. In anycase, currently available notch filters can be suitably employed forother, more widely separated combinations of wavelengths, or a futurenotch filter of improved design and performance could be employed withthe combination of wavelengths of FIG. 3B, 3C, 3D, or 3F.

Instead of, or in addition to, one or more thin film filters, one ormore Bragg filters can be employed for reducing transmission of theupconverting beam 22 to the imaging detector array 40. Bragg-type notchfilters typically exhibit spectral rejection bandwidths narrower thanthose of thin film notch filters. Suitable examples of such Braggfilters include, e.g., BragGrate™ Raman filters produced by OptiGrateCorporation, Bragg filters disclosed in U.S. Pat. No. 6,673,497 issuedto Elimov et al (which is incorporated by reference as if fully setforth herein), or crystalline colloidal Bragg filters such as thosedisclosed in Asher et al, Spectroscopy Vol. 1 No. 12 p. 26 (1986) (whichis incorporated by reference as if fully set forth herein). Such Braggfilters can provide adequate discrimination between the upconvertedimage beam 24 and the upconverting beam 20, even if relatively lowterahertz frequencies are employed that result in relatively closespacing of the upconverting spectrum and the upconverted image spectra(e.g.: separation of about 2 to 3 nanometers or less for terahertzfrequencies less than about 0.3 THz, e.g., as in FIG. 3F; or separationless than about 1 nanometer for terahertz frequencies less than about0.1 THz). Instead or in addition, other types of wavelength dependentfilters (e.g., such as those employed in DWDM optoelectronictelecommunications systems) can be employed for discriminating betweenthe upconverted image beam 24 and the residual upconverting beam 20.

A polarizer 39 of any suitable type can be employed if the beams 22 and24 are orthogonally polarized, instead of or (more typically) inaddition to the one or more filtering elements 38 (short pass, longpass, or notch). Any suitable one or more polarizers or one or morespectral filtering elements, extant or future developed, can be employedwithin the scope of the present disclosure or appended claims. Ininstances of relatively close spacing of the upconverting spectrum andthe upconverted image spectra, one or more polarizers 39, used insteadof or in addition to the one or more wavelength dependent filters 38,can provide adequate discrimination between the upconverting beam 22 andthe upconverted image beam 24. Whatever filtering arrangement isemployed, with spectral separation between the beams 22 and 24 of only afew nanometers or less, transmission of the upconverting beam 22 can insome instances be on the order of one part in 10⁶, one part in 10⁷, orone part in 10⁸, while in other instances the more desirable one part in10¹⁰ or one part in 10¹² can be achieved (as with the relatively largerspectral separations described above).

Note that even if only one upconverted image beam 24 is to be acquiredat the detector array 40, producing the DFG upconverted image beam 24can be advantageous. Each SFG photon is produced at the expense of acorresponding terahertz photon lost from the terahertz image beam 20;the intensity of the SFG upconverted image beam 24 is therefore limitedby the number of photons available in the terahertz image beam 20. Incontrast, each DFG photon produced in the upconverted image beam 24 alsoresults in a new photon produced in the terahertz image beam 20. Theintensity of the DFG upconverted image beam 24 is therefore limited bythe (much larger) number of photons available in the upconverting beam22. Consequently, if only one upconverted image is to be acquired, itmay be desirable to employ DFG to generate that upconverted image.However, generation of the DFG upconverted image beam 24 makes availableadditional photons in the terahertz image beam 20 for SFG. Even if theDFG upconverted image beam 24 is attenuated or blocked by the filter 38and only the SFG upconverted image beam 24 reaches the detector array40, generation of the DFG upconverted image beam 24 can increase thedetected intensity of the SFG upconverted image beam 24.

Note that simultaneous SFG and DFG described in the previous paragraphonly arises under certain conditions. In the examples described herein,the acceptance bandwidth of the quasi-phase-matched SFG and DFGprocesses is sufficiently large that both processes can occur with nearoptimal efficiency for the combination of λ_(UC)≈1064 nm, λ_(SFG)≈1058nm, and λ_(DFG)≈1070 nm shown in the examples of FIGS. 3B through 3D, orfor the combination of λ_(UC)≈1550 nm, λ_(SFG)≈1547.6 nm, andλ_(DFG)≈1552.4 nm shown in the example of FIG. 3F. For more widelyseparated SFG and DFG wavelengths (i.e., for higher terahertzfrequencies), or for a nonlinear optical medium with a smalleracceptance bandwidth, it may not be possible to produce both SFG and DFGupconverted image beams 24.

FIGS. 6A-6C show a cross-shaped aperture in a piece of sheet metal, anut, and a razor blade, respectively, that were imaged (as object 10; intransmission) using upconversion of a terahertz image. FIGS. 7A-7C arethe corresponding raw transmitted and upconverted images on the left andthe upconverted terahertz imaging beam 21 (without the object 10) on theright, and FIGS. 8A-8C are the corresponding normalized, upconvertedimages (normalized by dividing the raw upconverted image beam by theupconverted terahertz imaging beam). FIGS. 9A-9C show a razor bladecovered by adhesive tape, a leaf, and a piece of paper with water,respectively, that were imaged (as object 10; in transmission) usingupconversion of a terahertz image. FIGS. 10A-10C are the correspondingraw transmitted and upconverted images and FIGS. 11A-11C are thecorresponding normalized transmitted and upconverted images (normalizedby dividing the raw upconverted image beam by the upconverted terahertzimaging beam). The examples of FIGS. 6A-8C demonstrate upconversion oftransmitted terahertz images of objects that are opaque to terahertzradiation (i.e., upconversion of the terahertz “shadows” of suchobjects). The examples of FIGS. 9A-11C demonstrate upconversion oftransmitted terahertz images of objects having spatially varyingterahertz transmission (e.g., the veins in the leaf or the wet region ofthe paper) or that have features not discernable under opticalillumination (e.g., the razor blade concealed by the tape).

Another example is illustrated schematically in FIG. 12 of a system forgenerating and acquiring reflected and upconverted terahertz imagesusing so-called homodyne detection. In the preceding examples, theposition dependent intensity of the upconverted terahertz image dependson only the intensity of the terahertz image, i.e., theposition-dependent intensity of the upconverted image is substantiallyindependent of position-dependent phase of that image. In this example abeam splitter 44 is employed to split off a terahertz reference beam 23from the imaging terahertz beam 20. The beam splitter 44 combines theterahertz reference beam 23 with the terahertz image beam 21, which thenco-propagate through the nonlinear optical medium 36. The relative phaseof the terahertz reference beam 23 and the terahertz image beam 21 canbe varied by varying the length of a delay line, e.g., as shown in FIG.15. In this arrangement, the position-dependent intensity of eachupconverted image depends at least partly on the corresponding relativephase of the terahertz image beam and the terahertz reference beam.Acquiring terahertz images that include both intensity and phase at eachimage position potentially can yield more information regarding theobject 10 than image intensity alone. For example, a given object mightyield a featureless image if only intensity is detected, but mightexhibit image features manifested as phase variation across the image.Such an example is analogous to an object that is uniformly transparentto visible light but exhibits a spatially dependent index of refraction;an image consisting of only transmitted intensity would miss thatspatial variation.

In the homodyne detection arrangement of FIG. 12, the combined terahertzreference beam 23 and terahertz image beam 21 arrive at the nonlinearoptical medium 36 as a coherent superposition. The total terahertzintensity of the combined beams will include phase-independent portionsthat correspond to the squared amplitudes of the respective referenceand image beams, and will also include a phase-dependent portion thatcorresponds to cross terms involving both amplitudes. In one example,the total intensity arises from upconversion of an image beam combinedwith a reference beam 100 times more intense than the image beam.Interference of those beams results in phase-dependent intensityvariation of about ±20% of the reference intensity. This can be viewedas effectively amplifying the image beam, e.g., in some instances,depending on factors such as noise or detection sensitivity, a ±20%modulation of a non-zero background might be more readily detected andquantified than a nominally zero-background signal that is 100 timessmaller.

Homodyne detection can be employed using a single detector: the detectoris scanned across the upconverted image beam 24, and at each detectorlocation the delay line is scanned to vary the relative phase of theterahertz reference and image beams. Alternatively, an array detectorcan be employed, acquiring a complete image at each different relativephase. In either case, the resulting images can be presented orinterpreted according to standard methods for treating phase dependentquantities (e.g., using corresponding amplitude and phase images, orusing so-called “in-phase” and “quadrature” images, which might also bereferred to as real and imaginary parts of a complex-valued image).Homodyne detection techniques are widely employed in the field ofoptical coherence tomography; various numerical, computational, oranalysis methods developed in that field can be readily applied tohomodyne detection of upconverted terahertz images.

The configurations of FIG. 1, 2, or 5 can be modified to enableconvenient acquisition of images of the object 10 at other wavelengthsin addition to the upconverted terahertz images. For example, movableoptics can be employed to redirect the upconverting beam 22 to propagatealong the path of the terahertz imaging beam 21. The beamsplitter 34 andnonlinear optical medium 36 can be mounted so that they can be readilyremoved from the beam path, and the filter(s) 38 or polarizer 39 can beremoved or replaced as appropriate. A filter wheel can be employed, forexample, for swapping those elements into or out of the beam path. Inthis way, a given object can be imaged in place at differing wavelengths(e.g., 1.55 THz and 1064 nm) and then comparisons or correlations can bemade among those images. In addition, other wavelengths (in addition toλ_(UC)) that might be available can be used for imaging object 10 aswell.

In addition to the preceding, the following examples fall within thescope of the present disclosure or appended claims:

Example 1

A method for acquiring an upconverted terahertz image of an object, themethod comprising: (a) illuminating the object with a continuous-waveterahertz imaging beam characterized by a terahertz frequency betweenabout 0.05 THz and about 10 THz, a terahertz bandwidth, and a terahertzaverage power; (b) collecting at least a portion of the terahertzimaging beam, transmitted by or around the object or reflected orscattered from the object, and directing that portion to propagate as aterahertz image beam through a nonlinear optical medium, wherein theterahertz image beam is characterized by a terahertz image beam size atthe nonlinear optical medium; (c) directing a continuous-waveupconverting beam to propagate through the nonlinear optical medium,wherein the upconverting beam at least partly spatially overlaps theterahertz image beam in the nonlinear optical medium and ischaracterized by an upconverting wavelength, an upconverting bandwidth,an upconverting average power, and an upconverting beam size at thenonlinear optical medium; (d) upconverting, by nonlinear opticalinteraction of the terahertz image beam and the upconverting beam in thenonlinear optical medium, at least a portion of the terahertz image beamto form an upconverted image beam characterized by one or bothwavelengths produced by sum- or difference-frequency generation betweenthe terahertz image beam and the upconverting beam; (e) receiving atleast a portion of the upconverted image beam using an image detectorand detecting with the image detector an upconverted image formed at theimage detector by the upconverted image beam; and (f) allowing less thanabout 1 part in 10⁶ of the upconverting beam to reach the image detectorusing an image filtering element, (g) wherein the terahertz averagepower is greater than about 0.1 W, the upconverting wavelength isbetween about 400 nm and about 3500 nm, the upconverting bandwidth isless than about 0.1 nm, and the upconverting average power is greaterthan about 1 W.

Example 2

The method of Example 1 wherein the upconverting wavelength is betweenabout 1000 nm and about 1100 nm, the upconverting bandwidth is less thanabout 0.01 nm, and the upconverting average power is greater than about10 W.

Example 3

The method of Example 1 wherein the upconverting wavelength is betweenabout 1500 nm and about 1600 nm, the upconverting bandwidth is less thanabout 0.01 nm, and the upconverting average power is greater than about5 W.

Example 4

The method of any one of Examples 1 through 3 wherein a source of theupconverting beam is a solid state laser, a fiber laser, or asemiconductor laser.

Example 5

The method of any one of Examples 1 through 4 wherein: (i) the terahertzfrequency is less than about 3 THz and the terahertz average power isgreater than about 0.3 W, or (ii) the terahertz frequency is less thanabout 1.6 THz and the terahertz average power is greater than about 0.5W.

Example 6

The method of any one of Examples 1 through 5 wherein a source of theterahertz beam includes a backward-wave-type oscillator, one or moreterahertz amplifiers, or one or more harmonic generators.

Example 7

The method of any one of Examples 1 through 6 wherein the upconvertedimage wavelength is: (i) less than about 1 nm below or less than about 1nm above the upconverting wavelength, or both, (ii) about 1 to 2 nmbelow or about 1 to 2 nm above the upconverting wavelength, or both,(iii) about 2 to 3 nm below or about 2 to 3 nm above the upconvertingwavelength, or both, (iv) about 3 to 4 nm below or about 3 to 4 nm abovethe upconverting wavelength, or both, (v) about 4 to 5 nm below or about4 to 5 nm above the upconverting wavelength, or both, or (vi) about 5 to6 nm below or about 5 to 6 nm above the upconverting wavelength, orboth.

Example 8

The method of any one of Examples 1 through 7 wherein the imagefiltering element is arranged to allow: (i) less than about 1 part in10⁶ of the upconverting beam to reach the image detector; (ii) less thanabout 1 part in 10⁷ of the upconverting beam to reach the imagedetector; (iii) less than about 1 part in 10⁸ of the upconverting beamto reach the image detector; (iv) less than about 1 part in 10¹⁰ of theupconverting beam to reach the image detector; or (v) less than about 1part in 10¹² of the upconverting beam to reach the image detector.

Example 9

The method of any one of Examples 1 through 8 wherein the imagefiltering element includes one or more wavelength-dependent filters.

Example 10

The method of Example 9 wherein at least one of the one or morewavelength-dependent filters comprises a short-pass or a long-passfilter with a nominal cutoff wavelength between the upconvertingwavelength and one of the upconverted image wavelengths.

Example 11

The method of any one of Examples 9 or 10 wherein at least one of theone or more wavelength-dependent filters comprises a notch filternominally centered on the upconverting wavelength.

Example 12

The method of any one of Examples 1 through 11 wherein the nonlinearoptical medium is arranged so that polarization of the upconverted imagebeam is substantially perpendicular to polarization of the upconvertingbeam.

Example 13

The method of any one of Examples 1 through 12 wherein the upconvertingbeam and the upconverted image beam are polarized substantiallyorthogonally with respect to each other, and the image filtering elementincludes one or more polarizers arranged to substantially block theupconverting beam.

Example 14

The method of any one of Examples 1 through 11 wherein the nonlinearoptical medium is arranged so that polarization of the upconverted imagebeam is substantially parallel to polarization of the upconverting beam.

Example 15

The method of any one of Examples 1 through 14 wherein the nonlinearoptical medium is arranged so that the nonlinear optical interaction isa critically phase-matched process.

Example 16

The method of any one of Examples 1 through 14 wherein the nonlinearoptical medium is arranged so that the nonlinear optical interaction isa non-critically phase-matched process.

Example 17

The method of any one of Examples 1 through 14 wherein the nonlinearoptical medium is arranged so that the nonlinear optical interaction isa quasi-phase-matched process.

Example 18

The method of Example 17 wherein the nonlinear optical medium comprisesa periodically poled nonlinear optical crystal.

Example 19

The method of Example 17 wherein the nonlinear optical medium comprisesa stack of two or more optically contacted plates of a nonlinear opticalmaterial.

Example 20

The method of Example 17 wherein the nonlinear optical medium comprisesa stack of 6 to 12 optically contacted plates of GaAs about 300 μmthick, the terahertz frequency is about 1.55 THz, and the upconvertingwavelength is about 1064 nm.

Example 21

The method of any one of Examples 1 through 16 wherein the nonlinearoptical medium comprises a single GaAs plate up to about 8 to 9 mmthick, the terahertz frequency is about 0.3 THz, and the upconvertingwavelength is about 1550 nm.

Example 22

The method of any one of Examples 1 through 21 wherein (i) a firstfocusing element collects the portion of the terahertz imaging beam anddirects the terahertz image beam to propagate through the nonlinearoptical medium, (ii) the object and the nonlinear optical medium arepositioned at respective conjugate planes of the first focusing elementso that the terahertz image beam forms a terahertz image of the objectat the nonlinear optical medium, (iii) a second focusing elementcollects the portion of the upconverted image beam and directs theupconverted image beam to propagate to the image detector, and (iv) thenonlinear optical medium and the image detector are positioned atrespective conjugate planes of the second focusing element so that theupconverted image beam forms the upconverted image at the imagedetector.

Example 23

The method of any one of Examples 1 through 21 wherein (i) a firstfocusing element, characterized by an effective focal length f₁,collects the portion of the terahertz imaging beam and directs theterahertz image beam to propagate through the nonlinear optical medium,(ii) the object and the nonlinear optical medium are each positioned ata distance of about f₁ from the first focusing element so that theterahertz image beam forms a spatial Fourier transform of a terahertzimage of the object at the nonlinear optical medium, (iii) a secondfocusing element, characterized by an effective focal length f₂,collects the portion of the upconverted image beam and directs theupconverted image beam to propagate to the image detector, and (iv) thenonlinear optical medium and the image detector are each positioned at adistance of about f₂ from the second focusing element so that theupconverted image beam forms the upconverted image at the imagedetector.

Example 24

The method of any one of Examples 1 through 23 wherein the imagedetector comprises an imaging detector array, and detecting theupconverted image comprises receiving simultaneously different spatialportions of the upconverted image beam on multiple correspondingdetector elements of the imaging detector array.

Example 25

The method of any one of Examples 1 through 23 wherein the imagedetector comprises a single detector element, and detecting theupconverted image comprises scanning the single detector element acrossthe upconverted image beam so as to receive sequentially differentspatial portions of the upconverted image beam on the single detectorelement.

Example 26

The method of any one of Examples 1 through 25 whereinposition-dependent intensity of the upconverted image is substantiallyindependent of position-dependent phase of the terahertz image.

Example 27

The method of any one of Examples 1 through 25 further comprising:splitting off a portion of the terahertz imaging beam to form aterahertz reference beam; combining the terahertz reference beam and theterahertz image beam to co-propagate through the nonlinear opticalmedium; and acquiring multiple upconverted terahertz images withcorresponding different relative phases of the terahertz image beam andthe terahertz reference beam, wherein position-dependent intensity ofeach upconverted image depends at least partly on the correspondingposition-dependent relative phase of the terahertz image beam and theterahertz reference beam.

Example 28

An apparatus for acquiring an upconverted terahertz image of an object,the apparatus comprising: (a) a continuous-wave terahertz sourcearranged to illuminate the object with a terahertz imaging beamcharacterized by a terahertz frequency between about 0.05 THz and about10 THz, a terahertz bandwidth, and a terahertz average power; (b) one ormore terahertz optical components arranged to collect at least a portionof the terahertz imaging beam, transmitted by or around the object orreflected or scattered from the object, and to direct that portion topropagate as a terahertz image beam through a nonlinear optical medium,wherein the terahertz image beam is characterized by a terahertz imagebeam size at the nonlinear optical medium; (c) a light source arrangedto emit a continuous-wave upconverting beam; (d) one or more opticalcomponents arranged to direct the upconverting beam to propagate throughthe nonlinear optical medium, wherein the upconverting beam at leastpartly spatially overlaps the terahertz image beam in the nonlinearoptical medium and is characterized by an upconverting wavelength, anupconverting bandwidth, an upconverting average power, and anupconverting beam size at the nonlinear optical medium; (e) thenonlinear optical medium, wherein the nonlinear optical medium isarranged to upconvert, by nonlinear optical interaction of the terahertzimage beam and the upconverting beam in the nonlinear optical medium, atleast a portion of the terahertz image beam to form an upconverted imagebeam characterized by one or both wavelengths produced by sum- ordifference-frequency generation between the terahertz image beam and theupconverting beam; (f) an image detector arranged to receive at least aportion of the upconverted image beam and to detect an upconverted imageformed at the image detector by the upconverted image beam; and (g) animage filtering element arranged to allow less than about 1 part in 10⁶of the upconverting beam to reach the image detector, (h) wherein theupconverting wavelength is between about 400 nm and about 3500 nm, theupconverting bandwidth is less than about 0.1 nm, and the upconvertingaverage power is greater than about 1 W.

Example 29

The apparatus of Example 28 wherein the upconverting wavelength isbetween about 1000 nm and about 1100 nm, the upconverting bandwidth isless than about 0.01 nm, and the upconverting average power is greaterthan about 10 W.

Example 30

The apparatus of Example 28 the upconverting wavelength is between about1500 nm and about 1600 nm, the upconverting bandwidth is less than about0.01 nm, and the upconverting average power is greater than about 5 W.

Example 31

The apparatus of any one of Examples 28 through 30 wherein a source ofthe upconverting beam is a solid state laser, a fiber laser, or asemiconductor laser.

Example 32

The apparatus of any one of Examples 28 through 31 wherein: (i) theterahertz frequency is less than about 3 THz and the terahertz averagepower is greater than about 0.3 W, or (ii) the terahertz frequency isless than about 1.6 THz and the terahertz average power is greater thanabout 0.5 W.

Example 33

The apparatus of any one of Examples 28 through 32 wherein a source ofthe terahertz beam includes a backward-wave-type oscillator, one or moreterahertz amplifiers, or one or more harmonic generators.

Example 34

The apparatus of any one of Examples 28 through 33 wherein theupconverted image wavelength is: (i) less than about 1 nm below or lessthan about 1 nm above the upconverting wavelength, or both, (ii) about 1to 2 nm below or about 1 to 2 nm above the upconverting wavelength, orboth, (iii) about 2 to 3 nm below or about 2 to 3 nm above theupconverting wavelength, or both, (iv) about 3 to 4 nm below or about 3to 4 nm above the upconverting wavelength, or both, (v) about 4 to 5 nmbelow or about 4 to 5 nm above the upconverting wavelength, or both, or(vi) about 5 to 6 nm below or about 5 to 6 nm above the upconvertingwavelength, or both.

Example 35

The apparatus of any one of Examples 28 through 34 wherein the imagefiltering element is arranged to allow: (i) less than about 1 part in10⁶ of the upconverting beam to reach the image detector; (ii) less thanabout 1 part in 10⁷ of the upconverting beam to reach the imagedetector; (iii) less than about 1 part in 10⁸ of the upconverting beamto reach the image detector; (iv) less than about 1 part in 10¹⁰ of theupconverting beam to reach the image detector; or (v) less than about 1part in 10¹² of the upconverting beam to reach the image detector.

Example 36

The apparatus of any one of Examples 28 through 35 wherein the imagefiltering element includes one or more wavelength-dependent filters.

Example 37

The apparatus of Example 36 wherein at least one of the one or morewavelength-dependent filters comprises a short-pass or a long-passfilter with a nominal cutoff wavelength between the upconvertingwavelength and one of the upconverted image wavelengths.

Example 38

The apparatus of any one of Examples 36 or 37 wherein at least one ofthe one or more wavelength-dependent filters comprises a notch filternominally centered on the upconverting wavelength.

Example 39

The apparatus of any one of Examples 28 through 38 wherein the nonlinearoptical medium is arranged so that polarization of the upconverted imagebeam is substantially perpendicular to polarization of the upconvertingbeam.

Example 40

The apparatus of any one of Examples 28 through 39 wherein theupconverting beam and the upconverted image beam are polarizedsubstantially orthogonally with respect to each other, and the imagefiltering element includes one or more polarizers arranged tosubstantially block the upconverting beam.

Example 41

The apparatus of any one of Examples 28 through 38 wherein the nonlinearoptical medium is arranged so that polarization of the upconverted imagebeam is substantially parallel to polarization of the upconverting beam.

Example 42

The apparatus of any one of Examples 28 through 41 wherein the nonlinearoptical medium is arranged so that the nonlinear optical interaction isa critically phase-matched process.

Example 43

The apparatus of any one of Examples 28 through 41 wherein the nonlinearoptical medium is arranged so that the nonlinear optical interaction isa non-critically phase-matched process.

Example 44

The apparatus of any one of Examples 29 through 41 wherein the nonlinearoptical medium is arranged so that the nonlinear optical interaction isa quasi-phase-matched process.

Example 45

The apparatus of Example 44 wherein the nonlinear optical mediumcomprises a periodically poled nonlinear optical crystal.

Example 46

The apparatus of Example 44 wherein the nonlinear optical mediumcomprises a stack of two or more optically contacted plates of anonlinear optical material.

Example 47

The apparatus of Example 44 wherein the nonlinear optical mediumcomprises a stack of 6 to 12 optically contacted plates of GaAs about300 μm thick, the terahertz frequency is about 1.55 THz, and theupconverting wavelength is about 1064 nm.

Example 48

The apparatus of any one of Examples 28 through 43 wherein the nonlinearoptical medium comprises a GaAs plate up to about 8 to 9 mm thick, theterahertz frequency is about 0.3 THz, and the upconverting wavelength isabout 1550 nm.

Example 49

The apparatus of any one of Examples 28 through 48 wherein (i) the oneor more terahertz optical components include a first focusing elementarranged to collect the portion of the terahertz imaging beam and todirect the terahertz image beam to propagate through the nonlinearoptical medium, (ii) the object and the nonlinear optical medium arepositioned at respective conjugate planes of the first focusing elementso that the terahertz image beam forms a terahertz image of the objectat the nonlinear optical medium, (iii) the one or more opticalcomponents include a second focusing element arranged to collect theportion of the upconverted image beam and to direct the upconvertedimage beam to propagate to the image detector, and (iv) the nonlinearoptical medium and the image detector are positioned at respectiveconjugate planes of the second focusing element so that the upconvertedimage beam forms the upconverted image at the image detector.

Example 50

The apparatus of any one of Examples 28 through 48 wherein (i) the oneor more terahertz optical components include a first focusing element,characterized by an effective focal length f₁, arranged to collect theportion of the terahertz imaging beam and to direct the terahertz imagebeam to propagate through the nonlinear optical medium, (ii) the objectand the nonlinear optical medium are each positioned at a distance ofabout f₁ from the first focusing element so that the terahertz imagebeam forms a spatial Fourier transform of a terahertz image of theobject at the nonlinear optical medium, (iii) the one or more opticalcomponents include a second focusing element, characterized by aneffective focal length f₂, arranged to collect the portion of theupconverted image beam and to direct the upconverted image beam topropagate to the image detector, and (iv) the nonlinear optical mediumand the image detector are each positioned at a distance of about f₂from the second focusing element so that the upconverted image beamforms the upconverted image at the image detector.

Example 51

The apparatus of any one of Examples 28 through 50 wherein the imagedetector comprises an imaging detector array positioned and arranged toreceive simultaneously different spatial portions of the upconvertedimage beam on multiple corresponding detector elements of the imagingdetector array.

Example 52

The apparatus of any one of Examples 28 through 50 wherein the imagedetector comprises a single detector element arranged to be scannedacross the upconverted image beam so as to receive sequentiallydifferent spatial portions of the upconverted image beam on the singledetector element.

Example 53

The apparatus of any one of Examples 28 through 52 wherein one or bothof the one or more terahertz optical components or the one or moreoptical components are arranged so that position-dependent intensity ofthe upconverted image is substantially independent of position-dependentphase of the terahertz image.

Example 54

The apparatus of any one of Examples 28 through 52 wherein the one ormore terahertz optical components are arranged to split off a portion ofthe terahertz imaging beam to form a terahertz reference beam and tocombine the terahertz reference beam and the terahertz image beam toco-propagate through the nonlinear optical medium with differentrelative phase of the terahertz image beam and the terahertz referencebeam, and position-dependent intensity of each upconverted image dependsat least partly on the corresponding relative phase of the terahertzimage beam and the terahertz reference beam.

In the foregoing Detailed Description, various features may be groupedtogether in several example embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that any claimed embodiment requires morefeatures than are expressly recited in the corresponding claim. Rather,as the appended claims reflect, inventive subject matter may lie in lessthan all features of a single disclosed example embodiment. Thus, theappended claims are hereby incorporated into the Detailed Description,with each claim standing on its own as a separate disclosed embodiment.However, the present disclosure shall also be construed as implicitlydisclosing any embodiment having any suitable set of one or moredisclosed or claimed features (i.e., a set of features that are neitherincompatible nor mutually exclusive) that appear in the presentdisclosure or the appended claims, including those sets that may not beexplicitly disclosed herein. In addition, for purposes of disclosure,each of the appended dependent claims shall be construed as if writtenin multiple dependent form and dependent upon all preceding claims withwhich it is not inconsistent. It should be further noted that the scopeof the appended claims does not necessarily encompass the whole of thesubject matter disclosed herein.

For purposes of the present disclosure and appended claims, theconjunction “or” is to be construed inclusively (e.g., “a dog or a cat”would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat,or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or anytwo, or all three”), unless: (i) it is explicitly stated otherwise,e.g., by use of “either . . . or,” “only one of,” or similar language;or (ii) two or more of the listed alternatives are mutually exclusivewithin the particular context, in which case “or” would encompass onlythose combinations involving non-mutually-exclusive alternatives. Forpurposes of the present disclosure and appended claims, the words“comprising,” “including,” “having,” and variants thereof, wherever theyappear, shall be construed as open ended terminology, with the samemeaning as if the phrase “at least” were appended after each instancethereof, unless explicitly stated otherwise. For purposes of the presentdisclosure or appended claims, when terms are employed such as “aboutequal to,” “substantially equal to,” “greater than about,” “less thanabout,” and so forth, in relation to a numerical quantity, standardconventions pertaining to measurement precision and significant digitsshall apply, unless a differing interpretation is explicitly set forth.For null quantities described by phrases such as “substantiallyprevented,” “substantially absent,” “substantially eliminated,” “aboutequal to zero,” “negligible,” and so forth, each such phrase shalldenote the case wherein the quantity in question has been reduced ordiminished to such an extent that, for practical purposes in the contextof the intended operation or use of the disclosed or claimed apparatusor method, the overall behavior or performance of the apparatus ormethod does not differ from that which would have occurred had the nullquantity in fact been completely removed, exactly equal to zero, orotherwise exactly nulled.

In the appended claims, any labelling of elements, steps, limitations,or other portions of a claim (e.g., first, second, etc., (a), (b), (c),etc., or (i), (ii), (iii), etc.) is only for purposes of clarity, andshall not be construed as implying any sort of ordering or precedence ofthe claim portions so labelled. If any such ordering or precedence isintended, it will be explicitly recited in the claim or, in someinstances, it will be implicit or inherent based on the specific contentof the claim. In the appended claims, if the provisions of 35 USC§112(f) are desired to be invoked in an apparatus claim, then the word“means” will appear in that apparatus claim. If those provisions aredesired to be invoked in a method claim, the words “a step for” willappear in that method claim. Conversely, if the words “means” or “a stepfor” do not appear in a claim, then the provisions of 35 USC §112(f) arenot intended to be invoked for that claim.

If any one or more disclosures are incorporated herein by reference andsuch incorporated disclosures conflict in part or whole with, or differin scope from, the present disclosure, then to the extent of conflict,broader disclosure, or broader definition of terms, the presentdisclosure controls. If such incorporated disclosures conflict in partor whole with one another, then to the extent of conflict, thelater-dated disclosure controls.

The Abstract is provided as required as an aid to those searching forspecific subject matter within the patent literature. However, theAbstract is not intended to imply that any elements, features, orlimitations recited therein are necessarily encompassed by anyparticular claim. The scope of subject matter encompassed by each claimshall be determined by the recitation of only that claim.

What is claimed is:
 1. A method for acquiring an upconverted terahertz image of an object, the method comprising: (a) illuminating the object with a continuous-wave terahertz imaging beam characterized by a terahertz frequency between about 0.05 THz and about 10 THz, a terahertz bandwidth, and a terahertz average power; (b) collecting at least a portion of the terahertz imaging beam, transmitted by or around the object or reflected or scattered from the object, and directing that portion to propagate as a terahertz image beam through a nonlinear optical medium, wherein the terahertz image beam is characterized by a terahertz image beam size at the nonlinear optical medium; (c) directing a continuous-wave upconverting beam to propagate through the nonlinear optical medium, wherein the upconverting beam at least partly spatially overlaps the terahertz image beam in the nonlinear optical medium and is characterized by an upconverting wavelength, an upconverting bandwidth, an upconverting average power, and an upconverting beam size at the nonlinear optical medium; (d) upconverting, by nonlinear optical interaction of the terahertz image beam and the upconverting beam in the nonlinear optical medium, at least a portion of the terahertz image beam to form an upconverted image beam characterized by one or both wavelengths produced by sum- or difference-frequency generation between the terahertz image beam and the upconverting beam; (e) receiving at least a portion of the upconverted image beam using an image detector and detecting with the image detector an upconverted image formed at the image detector by the upconverted image beam; and (f) allowing less than about 1 part in 10⁶ of the upconverting beam to reach the image detector using an image filtering element, (g) wherein the terahertz average power is greater than about 0.1 W, the upconverting wavelength is between about 400 nm and about 3500 nm, the upconverting bandwidth is less than about 0.1 nm, and the upconverting average power is greater than about 1 W.
 2. The method of claim 1 wherein the upconverting wavelength is between about 1000 nm and about 1100 nm, the upconverting bandwidth is less than about 0.01 nm, and the upconverting average power is greater than about 10 W.
 3. The method of claim 1 wherein the upconverting wavelength is between about 1500 nm and about 1600 nm, the upconverting bandwidth is less than about 0.01 nm, and the upconverting average power is greater than about 5 W.
 4. The method of claim 1 wherein a source of the upconverting beam is a solid state laser, a fiber laser, or a semiconductor laser.
 5. The method of claim 1 wherein the terahertz frequency is less than about 3 THz and the terahertz average power is greater than about 0.3 W.
 6. The method of claim 1 wherein the terahertz frequency is less than about 1.6 THz and the terahertz average power is greater than about 0.5 W.
 7. The method of claim 1 wherein a source of the terahertz beam includes a backward-wave-type oscillator, one or more terahertz amplifiers, or one or more harmonic generators.
 8. The method of claim 1 wherein the image filtering element includes one or more wavelength-dependent filters.
 9. The method of claim 8 wherein at least one of the one or more wavelength-dependent filters comprises a short-pass or a long-pass filter with a nominal cutoff wavelength between the upconverting wavelength and one of the upconverted image wavelengths.
 10. The method of claim 8 wherein at least one of the one or more wavelength-dependent filters comprises a notch filter with a rejection bandwidth that includes the upconverting wavelength.
 11. The method of claim 1 wherein the upconverting beam and the upconverted image beam are polarized substantially orthogonally with respect to each other, and the image filtering element includes one or more polarizers arranged to substantially block the upconverting beam.
 12. The method of claim 1 wherein the nonlinear optical medium is arranged so that the nonlinear optical interaction is a quasi-phase-matched process.
 13. The method of claim 12 wherein the nonlinear optical medium comprises a stack of two or more optically contacted plates of a nonlinear optical material.
 14. The method of claim 1 wherein (i) a first focusing element collects the portion of the terahertz imaging beam and directs the terahertz image beam to propagate through the nonlinear optical medium, (ii) the object and the nonlinear optical medium are positioned at respective conjugate planes of the first focusing element so that the terahertz image beam forms a terahertz image of the object at the nonlinear optical medium, (iii) a second focusing element collects the portion of the upconverted image beam and directs the upconverted image beam to propagate to the image detector, and (iv) the nonlinear optical medium and the image detector are positioned at respective conjugate planes of the second focusing element so that the upconverted image beam forms the upconverted image at the image detector.
 15. The method of claim 1 wherein (i) a first focusing element, characterized by an effective focal length f₁, collects the portion of the terahertz imaging beam and directs the terahertz image beam to propagate through the nonlinear optical medium, (ii) the object and the nonlinear optical medium are each positioned at a distance of about f₁ from the first focusing element so that the terahertz image beam forms a spatial Fourier transform of a terahertz image of the object at the nonlinear optical medium, (iii) a second focusing element, characterized by an effective focal length f₂, collects the portion of the upconverted image beam and directs the upconverted image beam to propagate to the image detector, and (iv) the nonlinear optical medium and the image detector are each positioned at a distance of about f₂ from the second focusing element so that the upconverted image beam forms the upconverted image at the image detector.
 16. The method of claim 1 wherein the image detector comprises an imaging detector array, and detecting the upconverted image comprises receiving simultaneously different spatial portions of the upconverted image beam on multiple corresponding detector elements of the imaging detector array.
 17. The method of claim 1 further comprising: splitting off a portion of the terahertz imaging beam to form a terahertz reference beam; combining the terahertz reference beam and the terahertz image beam to co-propagate through the nonlinear optical medium; and acquiring multiple upconverted terahertz images with corresponding different relative phases of the terahertz image beam and the terahertz reference beam, wherein position-dependent intensity of each upconverted image depends at least partly on the corresponding position-dependent relative phase of the terahertz image beam and the terahertz reference beam.
 18. An apparatus for acquiring an upconverted terahertz image of an object, the apparatus comprising: (a) a terahertz source arranged to illuminate the object with a continuous-wave terahertz imaging beam characterized by a terahertz frequency between about 0.05 THz and about 10 THz, a terahertz bandwidth, and a terahertz average power; (b) one or more terahertz optical components arranged to collect at least a portion of the terahertz imaging beam, transmitted by or around the object or reflected or scattered from the object, and to direct that portion to propagate as a terahertz image beam through a nonlinear optical medium, wherein the terahertz image beam is characterized by a terahertz image beam size at the nonlinear optical medium; (c) a light source arranged to emit a continuous-wave upconverting beam; (d) one or more optical components arranged to direct the upconverting beam to propagate through the nonlinear optical medium, wherein the upconverting beam at least partly spatially overlaps the terahertz image beam in the nonlinear optical medium and is characterized by an upconverting wavelength, an upconverting bandwidth, an upconverting average power, and an upconverting beam size at the nonlinear optical medium; (e) the nonlinear optical medium, wherein the nonlinear optical medium is arranged to upconvert, by nonlinear optical interaction of the terahertz image beam and the upconverting beam in the nonlinear optical medium, at least a portion of the terahertz image beam to form an upconverted image beam characterized by one or both wavelengths produced by sum- or difference-frequency generation between the terahertz image beam and the upconverting beam; (f) an image detector arranged to receive at least a portion of the upconverted image beam and to detect an upconverted image formed at the image detector by the upconverted image beam; and (g) an image filtering element arranged to allow less than about 1 part in 10⁶ of the upconverting beam to reach the image detector, (h) wherein the upconverting wavelength is between about 400 nm and about 3500 nm, the upconverting bandwidth is less than about 0.1 nm, and the upconverting average power is greater than about 1 W.
 19. The apparatus of claim 18 wherein the upconverting wavelength is between about 1000 nm and about 1100 nm, the upconverting bandwidth is less than about 0.01 nm, and the upconverting average power is greater than about 10 W.
 20. The apparatus of claim 18 wherein the upconverting wavelength is between about 1500 nm and about 1600 nm, the upconverting bandwidth is less than about 0.01 nm, and the upconverting average power is greater than about 5 W.
 21. The apparatus of claim 18 wherein a source of the upconverting beam is a solid state laser, a fiber laser, or a semiconductor laser.
 22. The apparatus of claim 18 wherein the terahertz frequency is less than about 3 THz and the terahertz average power is greater than about 0.3 W.
 23. The apparatus of claim 18 wherein the terahertz frequency is less than about 1.6 THz and the terahertz average power is greater than about 0.5 W.
 24. The apparatus of claim 18 wherein a source of the terahertz beam includes a backward-wave-type oscillator, one or more terahertz amplifiers, or one or more harmonic generators.
 25. The apparatus of claim 18 wherein the image filtering element includes one or more wavelength-dependent filters.
 26. The apparatus of claim 25 wherein at least one of the one or more wavelength-dependent filters comprises a short-pass or a long-pass filter with a nominal cutoff wavelength between the upconverting wavelength and one of the upconverted image wavelengths.
 27. The apparatus of claim 25 wherein at least one of the one or more wavelength-dependent filters comprises a notch filter with a rejection bandwidth that includes the upconverting wavelength.
 28. The apparatus of claim 18 wherein the upconverting beam and the upconverted image beam are polarized substantially orthogonally with respect to each other, and the image filtering element includes one or more polarizers arranged to substantially block the upconverting beam.
 29. The apparatus of claim 18 wherein the nonlinear optical medium is arranged so that the nonlinear optical interaction is a quasi-phase-matched process.
 30. The apparatus of claim 29 wherein the nonlinear optical medium comprises a stack of two or more optically contacted plates of a nonlinear optical material.
 31. The apparatus of claim 18 wherein (i) the one or more terahertz optical components include a first focusing element arranged to collect the portion of the terahertz imaging beam and to direct the terahertz image beam to propagate through the nonlinear optical medium, (ii) the object and the nonlinear optical medium are positioned at respective conjugate planes of the first focusing element so that the terahertz image beam forms a terahertz image of the object at the nonlinear optical medium, (iii) the one or more optical components include a second focusing element arranged to collect the portion of the upconverted image beam and to direct the upconverted image beam to propagate to the image detector, and (iv) the nonlinear optical medium and the image detector are positioned at respective conjugate planes of the second focusing element so that the upconverted image beam forms the upconverted image at the image detector.
 32. The apparatus of claim 18 wherein (i) the one or more terahertz optical components include a first focusing element, characterized by an effective focal length f₁, arranged to collect the portion of the terahertz imaging beam and to direct the terahertz image beam to propagate through the nonlinear optical medium, (ii) the object and the nonlinear optical medium are each positioned at a distance of about f₁ from the first focusing element so that the terahertz image beam forms a spatial Fourier transform of a terahertz image of the object at the nonlinear optical medium, (iii) the one or more optical components include a second focusing element, characterized by an effective focal length f₂, arranged to collect the portion of the upconverted image beam and to direct the upconverted image beam to propagate to the image detector, and (iv) the nonlinear optical medium and the image detector are each positioned at a distance of about f₂ from the second focusing element so that the upconverted image beam forms the upconverted image at the image detector.
 33. The apparatus of claim 18 wherein the image detector comprises an imaging detector array positioned and arranged to receive simultaneously different spatial portions of the upconverted image beam on multiple corresponding detector elements of the imaging detector array.
 34. The apparatus of claim 18 wherein the one or more terahertz optical components are arranged to split off a portion of the terahertz imaging beam to form a terahertz reference beam and to combine the terahertz reference beam and the terahertz image beam to co-propagate through the nonlinear optical medium with different relative phase of the terahertz image beam and the terahertz reference beam, and position-dependent intensity of each upconverted image depends at least partly on the corresponding position-dependent relative phase of the terahertz image beam and the terahertz reference beam. 